It's a good concept and the execution is not too bad or too great. I liked your choice of audio as well. I also liked that I could stand on either side of a platform, although it took me a while to figure that out (due to the colour blending in with background)
A few suggestions that come to mind are-
1. Create a learning curve that is less steep- the level design could gradually increase in complexity rather than having challenging bits from the very beginning.
2. The background colour and the player bullets have very similar colours- it was hard to see where my bullets were going. You may want to make the bullets better visible like the ones enemies are shooting- those are bold, contrasting in colour, and thus very clear to see.
3. I think you forgot to mention in the controls that Space can be used for jump. I pressed it out of instinct and it worked!
Overall, it is has good potential; polishing it further can make it more enjoyable.
Congratulations on your first game jam, and nice work!